1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a wireless communication system, a base station and a data transmission timing control method capable of improving system throughput by transmitting data in parallel to the same mobile station from a plurality of base stations cooperating mutually.
2. Description of the Related Art
With an increase in communication speed in recent years, a MIMO (multiple input multiple output) technique capable of improving frequency utilization efficiency has been used in a wireless LAN or a mobile communication system. In a MIMO system, a transmitting station transmits data in parallel through a plurality of antennas, and a receiving station receives the data transmitted from the transmitting station with a plurality of antennas.
In a wireless communication system in which a transmitting station and a receiving station perform communication in a one-to-one manner, a singular value decomposition (SVD) beamforming transmission method has been proposed in order to maximize the communication capacity of a radio propagation channel. In the SVD beamforming transmission method, singular value decomposition is performed on channel matrix H indicating radio propagation path characteristics between the transmitting and receiving antennas. The matrix obtained by the singular value decomposition is weighted to a transmission vector signal and a reception vector signal in order to maximize the capacity of the propagation channel.
However, as the SVD beamforming transmission method needs to feed back channel information measured by the receiver side to the transmitter side, this method is not suitable for a communication environment in which a radio propagation channel varies greatly. The SVD beamforming transmission method is suitable, therefore, to improve the communication throughput in a wireless communication system provided that communication is performed in a quasi-static environment having little variation in the radio propagation channel, such as a wireless LAN system.
Many studies have been conducted on the communication capacity in terms of information theory, for example, in a one-to-N wireless communication system (BC: broadcast channel) in which one transmitting station communicates with a plurality of receiving stations and an M-to-one wireless communication system (MAC: multiple access channel) in which a plurality of transmitting stations communicate with one receiving station.
With respect to the communication capacity of BC, for example, dirty paper coding has been known as disclosed in “Writing on dirty paper” by M. Costa, IEEE Trans. Information Theory, Vol. 29, Issue 3, May 1983, pp. 440. It is proved that the dirty paper coding gives the upper bound of the system capacity, for example, by W. Yu and J. M. Cioffi, in “Sum capacity of Gaussian vector broadcast channels” IEEE Trans. Information Theory, Vol. 50, No. 9, pp. 1875-1892, September 2004.
However, a definite means for realizing the upper limit of the system capacity has not been known yet. In addition, the dirty paper coding is established only when a transmitter acquires the instantaneous channel information measured by a receiver. However, because large performance deterioration occurs due to a large channel variation during when the channel information measured by the receiver is fed back to the transmitter, it is difficult to realize the upper bound of the communication capacity of BC.
Further, it has been known that the communication capacity of BC is limited by the number of antennas provided in the transmitting station. A technique for performing dirty paper coding by cooperation among a plurality of transmitting stations so as to improve the overall throughput of a system is disclosed by S. Shamai and B. Zaidel, in “Enhancing the cellular downlink capacity via co-processing at the transmitting end”, Proceedings of IEEE Vehicular Tech. Conf., May 2001-Spring, pp. 1745-1749. However, the document premises an ideal communication system in which transmission and reception timings are completely synchronized with each other, but does not disclose a realizable cooperative control method taking a signal propagation delay, which occurs between a transmitting station and a receiving station during actual data communication, into consideration.